1. Field of the Invention
The present invention relates to a toner for developing an electrostatic charge image in electrophotograph, electrostatic recording, electrostatic printing and the like, a method for manufacturing the toner and an image forming method using the toner.
2. Description of the Related Art
In an electrophotographic image forming apparatus, an electrostatic recording apparatus or the like, toner is attached to a latent electrostatic image formed on a photoconductor, and the developed latent electrostatic image is transferred and fixed to a recording medium to form a toner image thereon. Further, a full-color image formation is to reproduce color in general by using four color toners of yellow, magenta, cyan and black. A full color image is formed by superimposing these color toners on a recording medium followed by heating treatment and fixation to the recording medium.
In general, toners used in developing an electrostatic charge image are color particles containing a coloring agent, a charge control agent and other additives in a binder resin. The manufacturing methods are largely categorized into pulverization methods and polymerization methods. In the pulverization method a toner composition is manufactured in which a coloring agent, a charge control agent, an off-set preventive agent and other agents are melted and mixed with thermoplastic resin in a state of homogeneous dispersion, and the toner composition is pulverized and classified to manufacture a toner. The pulverization method enables low-cost toner manufacturing to be achieved, but it suffers from a drawback that a broader particle size distribution is readily formed, which results in a very low yield upon classification. Further, it is difficult for the pulverization method to uniformly disperse such toner ingredients as a coloring agent and a charge control agent in thermoplastic resin. As a result, these ununiformly dispersed ingredients adversely affect the flowability, developing ability and durability of toner as well as image quality.
In recent years, there has been an increasing demand for higher-quality images that are comparable in resolution and definition to photographs and prints. As a method for manufacturing a toner with a smaller particle diameter and narrower particle size distribution, an emulsion polymerization method is employed that associate fine resin particles to obtain amorphous toner particles. However, toner particles obtained by the emulsion polymerization method may contain bubbles embedded therein that resulted from a surfactant. Therefore, when toner particles further aggregate and coalesce as they are, voids or voids are generated inside the aggregated toner particles, where surfactant and water remaining therein. This may result in problems such as reduced dielectric loss factor, poor charging capability, and background smear in the obtained image. Further, the remaining surfactant may contaminate the photoconductor, charging roller, developing roller and other members, affecting their inherent charging ability. There is also proposed a method of decreasing the void amount by addition of an anti-foaming agent, a metal compound-derived coagulant, fine resin particles or the like (refer to Japanese Patent Application Laid-Open (JP-A) No. 2000-275907). However, this proposal has a disadvantage that added agents affect toner quality and lead to increased costs.
There is also proposed a capsule-type toner prepared using an encapsulation method that involves uniformly encapsulating a coloring agent into particles without using a dispersion stabilizer (refer to JP-A No. 05-66600). However, depending on various conditions such as agitation condition upon phase inversion emulsification, it may result in voids in the resultant toner particles due to involvement of water, reducing the mechanical strength of toner. These toner particles with many voids therein are lower in mechanical strength and may lead to decreased dielectric loss factor and poor charging ability. In addition, toner particles may be crushed in the course of agitation inside a printing machine to generate a fine powder. This fine powder is responsible for adverse influences on a stable image supply, such as change in the charging ability, background smear and fogging due to reduced powder flowability and change in the charge level distribution. Further, there is proposed a method in which a specific organic solvent is used to swell a resin or resin portion of toner particles such that voids therein are reduced (see JP-A No. 08-62884). In this proposal, however, toner particles undesirably undergo surface morphology change by infiltration of organic solvent.
Since voids present inside a toner particle cause problems such as poor charging ability, durability or stability with time and background smears, conventional pulverized toner and polymerized toner both generally offer characteristics with their toner ingredients densely packed. With these toners, however, it is difficult to achieve a lower toner deposition mass per unit area (lower M/A) on paper or the like—a major effect brought about by reducing toner particle diameter—which leads to an increased number of copies per unit mass of toner. Therefore, there has been disclosed a method of attaining a lower toner deposition mass per unit area (lower M/A), while keeping the diameter of toner particles to a minimum extent and ensuring the developing property, transfer property, fixing property and the like. However, no description has been so far made about a method for controlling the quantity of voids (see JP-A No. 2005-17582).
Further, in a method for manufacturing the polymerized toner, it is important to attain a higher productivity, with the quality maintained. It is, therefore, required to control the void ratio inside the toner by using existing steps and processes, without providing any additional steps such as an agent adding step or a new step, and increase the number of copies per unit mass of toner, while keeping the charging property and durability.